Generally, an amplifier is an electronic circuit that generally increases, the strength of a signal. A measure of the amplification level often used is gain. Gain is typically measured in decibels (“dB”). Acceptable input ranges for an amplifier may include a range of signal levels to be amplified, and a range of frequencies to be amplified. Signals outside of the specified ranges may not be satisfactorily amplified.
Amplifying a signal can introduce distortion, and various figures of merit can be used to specify acceptable distortion levels in an amplifier. Amplifiers typically convert DC power to a replica of a signal applied to the amplifier. How efficiently this may be done typically depends upon amplifier bias that can place the quiescent operating point of the amplifier circuitry (typically one or more transistors) in various operating regions that typically trade off increasing distortion for increased efficiency.
Efficiency of an amplifier may be divided into classes. Class A amplifiers typically provide a faithful reproduction of the input but are typically inefficient, in the range of 10-20% with a theoretical maximum efficiency of 50%. Class B amplifiers have a better efficiency but are limited in application compared to Class A amplifiers because of higher distortion, such as crossover distortion. In practical audio power amplifier design, class AB designs are often used. Modern Class AB amps are commonly between 35-55% efficient with a theoretical maximum of 78.5%. Class C amplifiers are often used in transmitters to amplify radio signals. Class D switching amplifiers can have efficiencies above 95%. Class D amplifiers are operated with their active devices in on/off switching mode, reducing device dissipation by minimizing the time per RF cycle when both voltage and current are present in the device. Class D amplifiers typically produce an output waveform that converts an input waveform into a pulse-width modulated (square wave) analog signal with an amplitude equal to the supply voltage. The class E and class F amplifiers are highly efficient switching power amplifiers, typically used at such high frequencies that the switching time becomes appreciable compared to the duty time. Other classes of amplifiers are possible, as designers seek to improve amplifier performance, while seeking to reduce the resources needed by amplifiers to produce the improved performance.
Even though higher classes of amplifiers cannot generally be applied for linear amplification, these highly efficient amplifiers typically run much cooler (typically not needing cooling fans and the like to help remove heat) even in high power designs. Thus, high efficiency designs may be sought to be employed in designs where constant envelope signals are generated so that efficient use of power supplies and simplified heat exchanger designs may be utilized.
In popular use, the term amplifier today usually refers to an electronic amplifier, often as in audio applications or the like. A power amplifier (“PA”) typically increases voltage and current of an input signal.